1. Field of the Invention
The present invention generally relates to articles and devices for measuring the amount of particulates which have settled on a surface, and more particularly to an adhesive, gridded tape used to collect and analyze such particulates.
2. Description of the Prior Art
There are two primary applications of monitoring the air for particulate levels and distributions, namely, outdoor air pollution studies and indoor quality control. The Environmental Protection Agency (as well as its counterparts in other countries) has established maximum ambient levels for particulates of different sizes to insure the health and safety of individuals. The EPA usually requires impact studies before issuing a permit to build a facility which would introduce additional particulate pollutants into the atmosphere. Such studies include measurement of present particulate levels and projections of future levels after the planned facility is built and in operation.
Particulate control is also important in indoor applications, such as graphic arts houses and medical laboratories, and even more crucial in "clean rooms." Clean rooms are necessary for the fabrication of sensitive semiconductor components such as integrated circuits which are extremely susceptible to contamination by airborne dust and fibrils. Companies have gone to great lengths to minimize the presence of these fine particles, including the use of room air ionizers and filtration systems, but it is still necessary to monitor ambient particulate levels to insure proper quality control in the manufacture of the electronic components. Clean rooms are also used in non-electronic manufacturing facilities, such as in the manufacture of magnetic recording media.
There are two principle methods for sampling particulate pollutants, viz., filtration and gravimetric settlement. Filtration techniques usually require that a surface or object be washed with a liquid solvent, and the effluent collected and filtered (see ASTM test procedure F24-83). This procedure, however, has several drawbacks, such as the fact that fibrous filters are unsuitable for collecting fine particles as they tend to penetrate into the body of the filtering medium. Molecular or membrane filters can be used, but they are more expensive and necessitate the use of additional accessories. An example of a filtration device designed for measuring particulate contamination is the unit sold by Millipore Corp. of Bedford, Mass., under model number XX1004730.
The second method for sampling is by gravity or settlement, i.e., allowing particles suspended in the air to descend onto fallout jars, plates, glass slides, etc. Microscopic techniques may then be used to count the number of particles per specified area. In the past, this technique has also been employed using adhesive-coated paper. For example, an established method for analyzing the amount of wind-blown particles as a function of wind direction requires that an adhesive strip be annularly mounted on the surface of a cylindrical holder.
Settlement techniques are, nonetheless, still crude with respect to determination of the amount of particles per unit area. One rudimentary solution is to cut the strip into squares of unit area (e.g., one square inch), and then count the number of particulates on each square, arriving at an average value. The introduction of the cutting step, however, provides room for further human error. An alternative method requires the use of a complicated device known as a Gruber Comparator, in which photographic standards are compared to the collected sample. Use of such a device is undesirable inasmuch as 1) it involves extra expense, 2) it requires previous calibration of the photographic standards, and 3) it requires a subjective comparison of visual images. It would, therefore, be desirable and advantageous to devise an article which would simplify the collection and measurement of suspended particulates.
Accordingly, the primary object of the present invention is to provide an article for monitoring ambient particulate levels.
Another object of the invention is to provide such an article which may easily be used without any special training.
Still another object of the invention is to provide a particulate collection device which simplifies determination of the number of particulates in a given area.